I'm not an expert by any stretch of the imagination but I am dealing with three typs of repairs. The first type are nicks in the trailing edge. Both rudders have this issue when the tiller crossbar flipped backwars during a wipe out and landed on the back of the rudders. The indentions into the trailing edge were less than a quarter of an inch deep. To repair I used a dremel with a carbid tip and removed the loose or chipping off material. I then light worked the perimeter to create a slope from the damage portion to the undamaged portion. I removed very little material during this stage. Given the small amount to be rebuilt, and the fact that these areas are not stressed, I fill the craters with epoxy resin thickened with cobosil and high density filler. I will describe the gelcoat portion in a moment as it applies to all the repair types I am addressing. I also had one of these types of repairs to make to the daggerboards trailing edge which I was nervous about because it is clear carbon fiber on the trailing edge with that pretty patern, and it is black. For this I used the same method except I was more stringent on how much material I removed in the prep, and I added graphite powder to the mix to make the final product black. There will be no gelcoat at this repair but the finish techniques are the same, this repair looks awesome and you can hardly tell it has been done unless you are inches from the board.
The second type of repair is a nick, typically very shallow and only affecting one side of the board. For these I want to simply fill the "divot" with gelcoat and move one.
The third repair type I am fortunate not to have on this go around but I did on my old Hobie 18 where the trailing edge was quite eroded away, 1/4 to 1/2 inch I would say. In this type of repair I would clean the loose stuff away as before. Than using the dremel and fiber board I would cut a slit in the center of the trailing edge where the damage is located. I would than take a cured peice of fiberglass mat and epoxy it into the slit. This than become the backer for filling as described earlier.
As for the finishing of gelcoat I personally like to spray my applications. Sand the repair and immediate area with 180-220 grit, clean well and spray the gelcoat. For small repair like this the cheap prevail sprayers available at the box store for 5 bucks are great. I thin my gelcoat with duragloss made by duratec. Once cured fair in the repair with the surrounding area using the 220, than drysand with 300, 400, and 500. Then wetsand with 600, 800, 1000, 1500, 2000, and finally buff with ultra fine rubing compound such as 3M Perfection. Each sanding step is very breif, used only to remove the scratches made by the previous paper. Assuming your gelcoat is the correct tint (there are many many shades of white), your repair should be pretty unnoticeable.
